It is known to produce three-dimensional images of objects from two dimensional images displayed on a cathode ray tube (CRT) screen by sequentially displaying depth planes or "slices" of an object on the screen on a frame-by-frame basis. The slices are projected onto a moving screen which is synchronised with the screen images such that the projected slices appear at the appropriate point in space relative to each plane of the original object. Persistence of vision enables a viewer to see an apparently solid but translucent object constructed from the various slices. Examples of such systems are described, for example, in U.S. Pat. No. 4,607,255 (Fuchs et al), U.S. Pat. No. 4,462,044 (Thomason et al), UK Patent Application No. 2 000 680 A (Sher) and European Patent Application No. 0 271 650 A1 (Muckerheide).
In these existing systems the slice images are created by conventional raster displays, that is each image is created in a raster frame made up of a large number of raster lines with selected points or spots on selected lines being illuminated such that the spots collectively create the desired image on the screen. The creation of each raster frame requires a relatively long time period (16.67 mS at a 60 Hz refresh rate) such that, in practice, it has proved difficult to produce the number of slice images necessary to create a three-dimensional image of any depth or resolution within the short period necessary to create a solid non-flickering image. Proposals have been made to increase the frame or refresh rate to a higher level (such as 120 Hz) to overcome this problem, however the volume of data which must be processed to produce this number of frames requires provision of relatively sophisticated, and therefore expensive, computer systems.
These difficulties do not arise where an actual object is available, and there have been a number of examples in which reflected three-dimensional images of real objects have been created. In Research 1960, Gregory used a moving lens to focus sharply through each of the planes of an object and was able to view the resulting solid images on a revolving spiral mirror. However, the most pressing need is for a system which will display electronically created or processed images in three-dimensions.
Three-dimensional viewing systems utilising head mounted displays (HMDs) are available, however these produce small images of relatively poor resolution and have a restricted field of view. Further, as the HMD must be fixed relative to the viewer's head it is not possible to view a "stationary" image from different angles, and the image must be viewed in isolation from other viewers.
Accordingly, there is a need for a system which will permit computer generated objects to be viewed in three-dimensions and occupy a real image space. Such a system would be particularly useful in computer aided design (CAD), allowing designers to view a real image of, for example, a computer generated motor vehicle body.